1. Field of the Invention
The present invention relates to video display systems, and more particularly relates to the reduction of electromagnetic interference ("EMI") emission from video display systems employing cathode ray tube ("CRT") displays.
2. Art Background
A. CRT Monitor Background
In a conventional CRT video display monitor, images are formed by scanning a beam of electrons across a photon emitting surface according to an input video signal. In a color CRT display, three primary color video signals drive three primary color electron guns, each gun emitting a stream of electrons which subsequently impinges on its particular color phosphor. Brightness of individual phosphors is controlled by the number of electrons impinging on that phosphor according to the input video signal for that color. In conventional CRT color displays, color images are formed by the composition of light emitted by individual red, green, and blue phosphors, which when viewed from a distance, appear to form a full color image. In a color CRT display, electrons from the red cathode, or electron gun, are accelerated towards the CRT screen by the high voltage of the anode. When the electrons emitted from the red cathode strike a red phosphors, red light is emitted. Electron beam deflection plates around the neck of the CRT deflect the electrons from the red cathode such that the electrons strike only the red phosphors. The deflection can be achieved by fixed permanent magnets, electromagnets, or by electrostatic deflection coils. Similarly, green and blue light is created when electrons from the green and blue electron guns are accelerated towards the CRT screen by the high voltage of the anode, whereupon they strike the green and blue phosphors respectively.
The position of an object to be displayed on the CRT screen is a function of both the amount of deflection of the electron beam for each primary color comprising the color image and the time when the video signal appears at the video input to the display. Each variable is independent of the other. Changing the amount of deflection of the electron beam will move all images of the color pertaining to that electron beam simultanesouly. Changing the time when a video signal comprising an object to be displayed on the screen appears at the video input will result in movement of only that object. Because the electron beams for all primary colors red, green and blue scan from top to bottom and from left to right, video signals for an object that occurs later in time will appear either further to the right or lower to the bottom of the CRT display relative to another object occurring earlier.
B. Electromagnetic Interference ("EMI") Background
Electromagnetic Interference, or EMI, may derive from several sources. Each component part of a video display system contributes a portion of the total EMI generated by the entire video display system. For example, a video generator generally must be equipped with some openings for cooling ventilation, which results in some radio frequency ("RF") radiation escaping from the video generator enclosure. Additionally, internal digital logic operating at 5 volt peak-to-peak signal levels contribute to the total EMI, although the final video output from the video generator is typically only 0.7 volts peak-to-peak. The video generator is connected to other component parts of the video display system including the CRT display, via cabling. To the extent that cables are unshielded, RF energy will radiate from the cable further contributing to the measurable EMI for the display system.
The greatest contributor to total EMI for video display system is the video monitor. Principally, the signal level operating within the electron gun cathodes is more that 50 times the signal level within the interconnecting cabling. The magnitude of EMI generated is directly proportional to amplitude of the source signal. Thus, the video monitor, with a substantially higher signal level, will contribute a substantially higher proportion of the total EMI. Additionally, because of the higher power consumption and dissipation within the monitor, there is an increased need for cooling ventilation, requiring additional ventilation openings and/or fans to move air through the monitor enclosure. Finally, because the CRT screen must be "open" to permit viewing the display, EMI emissions will be higher than if the CRT electron guns and accelerating anode were entirely enclosed. Some shielding may be provided within a CRT display, but generally compromise is required resulting in measurable EMI emission from the display screen. Shielding and filtering are commonly used and well known techniques in the art for reducing EMI emissions in display monitors. In the prior art, every radiating circuit is placed within singly or multiply shielded metal enclosures. Moreover, every wire which passes from the shielded component to the outside environment must be filtered using RLC circuits or other dissipative elements such as ferrite chokes, depending on the frequency of the radiating mechanism. Typically, AC power lines and user controls must be filtered in accordance with the above methods. Depending on the permissible levels of EMI for a particular application, reduction of EMI with known filtering and shielding techniques can result in significant added expense to the cost of the CRT display used in a video display system.
C. Comb Filter Background
In conventional electrical filters, attenuation of a signal is dependent upon the frequency of the signal being filtered. Such conventional filters are implemented with frequency dependent components such as capacitors and inductors. For example, a low pass filter employing a capacitor and a resistor will permit frequency components below the cutoff frequency to pass unattenuated, and will attenuate signal components above the cut off frequency. Alternatively, another type of frequency dependent filter known in the prior art is a transversal filter. A transversal filter consists of a delay line with signal taps at various points along the delay line, as shown in FIG. 1. The transversal filter relies upon the propagation time delay for a signal transmitted along the line to provide the frequency discrimination function within the filter. For example, assume a signal is applied to a 2 microsecond delay line having taps at the midpoint and at the end. Where the taps are added together in a summing network, the output will be a composition of the signals derived from the mid point tap and the terminal tap. If a 1 MHz sine wave is applied to the input of the delay line, because the period of a 1 MHz sine wave is 1 microsecond, the phase of the input signal at the first tap is exactly the same as the phase of the signal at the second, or terminal, tap at the end of the two microsecond delay line. When the two taps are summed together, the output signal will be the same phase as the input signal with twice the amplitude. If, instead, a 500 KHz signal is applied to the input of the delay line, because the period of the 500 KHz signal is 2 microseconds, the signal at the first tap is 180 degrees out of phase with the signal at the end of the delay line. Summing the signal at the first tap and at the terminal tap results in a zero output from the summing network. Accordingly, if input signals of all possible frequencies are applied to the input of the filter, a series of nulls will be seen at 500 KHz, 1.5 MHz, 2.5 MHz, 3.5 MHz etc., where the signals taken from the first tap and from the terminal tap are exactly out of phase. Conversely, at 1 MHz, 2 MHz, 3 MHz, etc., the output signal will be at a maximum because the signals taken from the first tap and the terminal tap are exactly in phase. The result shown in FIG. 1 is a response spectrum which has the appearance of a comb, hence the term "comb" filter, where the "teeth" are the nulls of the filter. Depending on the number of taps in the delay line, the number of nulls and the frequency at which nulls occur will vary. A representative discussion of comb filters is given in Bensen, Television Engineering Handbook 13:149-152 (1986).
As will be discussed below, the present invention provides simple, inexpensive, yet effective apparatus and methods for reducing the EMI emitted from a CRT display by time delaying the component color video signals comprising a video image to be displayed. Introduction of time delayed video signals in turn introduces signal nulls depending on frequency of the video signals comprising the video image, resulting in lower overall EMI emission for the CRT display.